This is the current and most robust design for the main board. There's been great help in creating this, both here on GitHub, but also at the Norwegian home automation forum, www.hjemmeautomasjon.no

The design uses an ESP8266 in an ESP12 module from AI Thinker for micro processing and the TSS721 from Texas Instruments for level conversion from M-Bus to TTL (3.3V). An earlier design solved the level conversion using an op-amp, which might be an easier and more available solution, but also more error-prone.

The board

uses TSS721 for MBus to TTL conversion
holds ESP8266 for processing and transmit HAN data to WiFi / MQTT
full schematics and PCB in editable KiCad source files
features a temperature sensor (DS18B20)
uses a modular design, leaving these features optional
- temperature sensor
- M-bus TX
- WiFi (Leave ESP, power supply and use for only M-bus to TTL)

Full schematics is available for this board in the kicad folder.

There is also a 3D printable enclosure in stl folder.

The completed board mounted in a 3D printed enclosure

...installed with the electrical meter...